Dai Greene, the reigning world 400m hurdles champion, has revealed that he has not jumped a hurdle for three weeks because of a calf injury but insists he will be fit to defend the title in Moscow.

Greene, who missed a Diamond League event in Monaco and the Anniversary Games in London after suffering a small tear in a calf just after the UK trials in July, is back running again and hopes to start hurdling by the end of this week. The first round of the 400m hurdles is on Monday.

But the Welshman has conceded that he has no idea what shape he will be in for the world championships, or whether he has a realistic hope of retaining the title. "It's been a really depressing last few weeks but hopefully we'll manage to get rid of the problem and I'll be on the start line," he said. "I'll do everything I can to do that. But we haven't let ourselves look realistically what we can achieve because we don't really know."

Greene has been keeping fit by aqua jogging, bike sessions and lifting weights in the gym, but he concedes "there is no substitute for running".

"I guess I won't know exactly how it feels until I race on the Monday but I want to give it the best shot I can," he added. "It's not ideal preparation. I've had my best winter ever behind me so it's just frustrating."

Greene has denied there is a possibility he might withdraw, but his frustration is obvious and understandable. Last year he finished fourth at the Olympics after undergoing surgery on his left knee the previous winter but coming into the 2013 season he felt he was in the form of his life only for injury to intervene with the world championships round the corner.

"I was in really good shape at the UK trials and I felt I could have run a lot faster than 48.66sec if I'd been in a race that was more competitive," he said. "I hope that I won't have lost too much. It's very much heading into the unknown. I know I've got the ability to get to the final and be competitive so hopefully it's still there for me."

Greene travels to Moscow admitting that he is angered by more reports of doping, the latest of which saw 31 Turkish athletes banned for two years for positive tests, but says he does not believe it will harm the sport in the long term.

"Cycling had its horrible moment for a few years and it culminated with Lance [Armstrong], and athletics seems to be heading the same way, but cycling came out the other side and is booming in the UK so hopefully athletics will come out stronger from all the negative press it's getting."

But when asked whether athletics could be about to undergo an Armstrong moment, with the sport cleaning up its act, Greene sounded sceptical. "It could well be. Who knows? You wouldn't be surprised given the last few months if there were even more chapters to the story."

Greene also hit out at the growing number of athletes, including the former 100m world record holder Asafa Powell, who have blamed hidden substances in supplements for their positive tests.

"A small minority who fail tests are in that situation but it is an all-too-common excuse," he said. "People have to be responsible for what they take and I run everything by UK Athletics because I don't want to make a silly mistake that means I can't run for a few years. That would be devastating for me.

" You have to be responsible for that yourself," he added. "It's an easy cop-out to say it was contaminated or you didn't read the label properly. It's just foolish. You know you can get tested any day, so its silly to think: 'Oh, I'll just buy this and I'll be fine'. It's just a bit of a soft excuse, really."